1. Field of the Invention
This invention relates to a radiation image processing method and apparatus therefor, and more particularly to a method of and apparatus for conducting an unsharp masking process on an image on a radiographic film (hereinafter referred to as "X-ray image") to improve the diagnostic accuracy when the X-ray image is duplicated, and further to a method of and apparatus for frequency processing an image in a system for recording and reproducing an radiation image by use of stimulable phosphor sheet.
2. Description of the Prior Art
Since X-rays are harmful to human bodies, it is desired to obtain as much information as possible by one exposure to X-rays. In general, X-ray films should have sensitivity which is high enough and latitude which is wide enough to record radiographic images together with contrast which is high enough, sharpness which is high enough and granularity fine enough to satisfy the requirements for diagnostic observation. However, these properties cannot generally be obtained at the same time due to their contradicting conditions. Therefore, a certain compromise between these requirements is practically made in preparing X-ray films.
Hence, it has been desired to improve the contrast, sharpness and granularity by image processing the X-ray image; by reading out the X-ray image and converting the image into an electric image signal, processing the signal, and reproducing a processed image with improved properties. By such an image processing, it becomes possible to enhance the quality and diagnostic accuracy of the X-ray image, to obtain as much diagnostic information as possible, and further to provide X-ray films with further improved recording properties such as sensitivity and latitude.
On the other hand, it has been known in the art (as disclosed in Japanese Unexamined Patent Publication No. 48-25523) to use a photographic film having a two-step gradient contrast in which the gradient is partly high and partly low and to conduct an unsharp masking process which emphasizes frequencies in the high spatial frequency region. This process is employed in a system in which large sized X-ray film images are duplicated on a reduced scale on small sized X-ray film to save the storage space and at the same time to maintain the high quality of the image.
This system, however, has a drawback in that noise is also increased because the high frequency component of the image signal is emphasized. Accordingly the diagnostic accuracy and image quality of the reduced scale image are not improved.
U.S. Pat. No. 4,317,179 discloses an X-ray image processing method to solve this problem. It was found that an important frequency to be emphasized is extremely low and that the conventional methods which emphasize high frequency components to improve the sharpness also deteriorate diagnostic accuracy by emphasizing noise. Accordingly, diagnostic accuracy was improved by lowering the emphasis of high frequencies and emphasizing the super-low frequency component.
The above-mentioned method of emphasizing the super-low frequency component obtains a high quality image which is more acceptable to visual sense and in which high frequency noise is comparatively decreased. In this method, an original X-ray image is scanned and converted to an electric signal and the electric signal is subjected to an image processing operation in which an unsharp masking operation described by: EQU D'-Dorg+.beta.(Dorg-Dus) (1)
is conducted (wherein D' is the density of the final X-ray image, Dorg is the density of the original image, .beta. is an emphasizing coefficient, and Dus is an unsharp mask density corresponding to the super-low frequency at the respective scanning point). Frequencies lower than the super-low frequency are thus emphasized.
However, in this method there some drawbacks have been discovered:
By emphasizing this particular frequency component, edges of objects in the final image are emphasized, regardless of the size of the objects as is the part of the image where a larger structured object overlaps a smaller structured body. Further, even in the area other than the overlapping part, overshooting or undershooting, ghost images to appear at the edges of objects in the final image. This will, of course, affect a diagnosis.
The above-mentioned drawbacks inherent in said prior art will now be described in more detail with reference to the drawings, particularly to FIGS. 1A and 1D which are an example in one dimension. In FIGS. 1A and 1D, the abscissa represents time (position) and the ordinate represents the level of the signal. According to formula (1), the unsharp mask density Dus as shown in FIG. 1B is subtracted from the original density Dorg as shown in FIG. 1A to obtain Dorg-Dus, which is multiplied by .beta. to obtain Dorg-Dus as shown in FIG. 1C. This is added to the original density Dorg to obtain an image signal having emphasized edges with overshoot A or undershoot B as shown in FIG. 1D. In this prior art method, there is an advantage: the contrast of an object is enhanced when the size of the unsharp mask is larger than the size of the object; but edges are also undesirably emphasized. The emphasis of the edges and depends upon the gradient of the edge of the object and occurs regardless of the size of the object. Thus the edges of bones or the heart are emphasized, even though these structures have large size, because they also have steep edges. The emphasized edges have overshoot or undershoot and appear as a ghost image which deteriorates diagnostic accuracy and image quality.
Various patents, for example, U.S. Pat. Nos. 4,258,264 and 4,276,473, provide a method and apparatus for recording and reproducing a radiation image of a human body or the like by use of a stimulable phosphor. In radiation image recording and reproducing system using a stimulable phosphor, the image signal can be subjected to various kinds of image processing and accordingly, is effective for enhancing the diagnostic efficiency and accuracy when it is applied to medical diagnosis.
One of the image processing methods which can be applied to enhancing the diagnostic efficiency and accuracy is a frequency processing as suggested in U.S. Pat. No. 4,315,318. This frequency processing is a method of emphasizing a particular frequency component by use of an unsharp masking process quite similar to said unsharp masking process as mentioned with reference to FIGS. 1A and 1D. In place of Dorg, Dus and D', an original signal Sorg, unsharp mask signal Sus and reproduced signal S' are employed. Accordingly, this method will also suffer from the drawbacks caused by the ghost image.